The present invention relates to the manufacture of semiconductor integrated circuits, and more particularly to apparatus for ultra high purity plasma cleaning and ultra pure material epitaxy.
Ion bombardment of surfaces is an important aspect of plasma etching, reactive ion etching, and sputter deposition. Ion bombardment also is an important part of several analytical techniques such as secondary ion mass spectrometry (SIMS) and low energy ion scattering spectroscopy (LEIS). Auger electron spectrometry (AES) also employs low energy inert gas ions for physical sputtering to obtain erosion to support depth profiling. Although these have become very important processes, the ion induced surface chemistry and alteration of near surface chemical, physical and electronic characteristics are very complex and remain poorly understood processes. This has led to the empirical development of most commercial plasma assisted etching processes.
Plasmas are partially ionized quasi-neutral gases. They can be created in a vacuum chamber by applying enough electric field to ionize the gases. The power source may be a DC electric field, inductive RF coil, microwaves or a capacitively coupled RF electric field. Electrons have small mass relative to other particles so most of the energy gained in these systems is initially absorbed by the electrons. These high energy electrons collide with other particles and ionize the gas and sustain the plasma. The typical ionizing potential is high so that the majority of molecules and atoms stay neutral. Eventually a DC potential will build up between a plasma and any dielectric surface nearby, preventing any further imbalance.
In general, semiconductor processing plasmas are in a state of thermal non-equilibrium and are affected by the following: (a) Power--By increasing power absorbed the sheath potential is increased as well as the number of ions produced. Any wafer within the plasma will experience induced temperature increases from increased ion energy bombardment as well as increased ion flux. Obviously, more damage can be done to the substrate at higher powers.
(b) Pressure--At higher pressures, more gas molecules are available which is generally believed to result in higher ion flux. However, in our work we discovered a very sharp peak in production of excited hydrogen species at low pressure in our electron cyclotron resonance, (ECR) plasma generator. This discovery has enabled us to take advantage of this peak in obtaining very fast etching at low ion energy so that we avoid surface damage usually occurring with energetic ions. While such surface damage is expected with heavy ions, it has also been documented with light ions such as H.sup.+.
(c) Device configuration--In ECR devices, parameters such as chamber geometry including positioning of the substrate, magnet configuration chamber materials and ion density uniformity will affect the etching process.
It is postulated that the explanation for the distinctly different etching rates of materials by various etchant gases has to do with the ability of the reactive gas molecule to penetrate into the surface being etched and to break the subsurface bond or lower the binding energy for the surface atoms and to replace that bond by bonding itself to the subsurface such that the released product is volatile at the temperature of release. It is believed that when an energetic ion strikes a solid, it transfers its energy to near surface atoms through a series of elastic collisions and electronic and vibrational processes. Collisional cascade effects can produce ion implantation, crystalline damage, ion mixing and physical sputtering. These effects can also result from low energy ion bombardment. Ion mixing is the process under which target atoms are relocated by ion impact, which process is broken up by recoil and cascade contributions. It is believed that mixing processes may be important in enhancing volatile product formation. This is distinct from sputtering in which near surface atoms receive enough momentum transfer perpendicular to the surface to overcome the surface potential barrier and thereby escape into the vacuum.
Two types of equipment are being most frequently used for plasma etching. One type of such apparatus employs a resonant cavity excited by RF fields to induce and sustain the plasma. These are relatively simple devices but a difficulty that they present is plasma density and ion energy cannot be separately controlled. Accordingly high ion energies cannot be curtailed in high power requirement situations. The conventional RF field, inductive or capacitively coupled, is characterized by low electron density and high plasma potential. Another type of plasma producing apparatus employs a microwave resonant cavity to produce plasma which is flowed out of the cavity to a remote area. This is called a downstream microwave plasma. Downstream microwave plasma is characterized by low electron density, low plasma potential, and high plasma pressure. For this reason, electron cyclotron resonance (ECR) microwave plasma apparatus have become more popular for etching application. The ECR apparatus can provide low pressure, high ion density at low ion energy and the ion energy can be controlled by substrate bias and the plasma potential is also low.
An electron in motion in a magnetic field is acted upon by the field to produce a force at right angles to the direction of motion of the electron. As a result, an electron entering a fixed magnetic field will follow a curved path. The radius of curvature is an inverse function of the intensity of the magnetic field. The frequency of electron rotation, w, is expressed as w=2.8.times.10.sup.6 B cycles/see where B is in gauss. This is known as the electron cyclotron resonance frequency. We have designed our ECR plasma generator to employ a magnetic field of 875 gauss and the corresponding cyclotron frequency of 2.45 GHz.
In recent years, the demands for reducing line widths and increasing device density in integrated circuits has forced the industry towards a manufacturing device called the integrated cluster tool. The integrated cluster tool is a multichamber vacuum system, in which the working chambers are arranged around a central transfer chamber and in which each working chamber is separated from the central transfer chamber by a pair of gate valves forming a vacuum lock. During operation, a semiconductor wafer can be processed in a working chamber, while the remainder of the cluster apparatus is isolated from the environment of any of the other working chambers. After a wafer treatment is completed in a particular working chamber, the wafer is able to be automatically passed to the transfer chamber through a double gate valve and then automatically passed to a subsequent working chamber through another double gate valve. This integrated cluster tool permits a plurality of various vacuum working chambers to be "clustered" around the central transfer chamber and permits the processing of a wafer through many of its most demanding processes without any requirement for the wafer to be passed back into ambient or clean room air. It has been proven that it is impractical and almost impossible, to control the particulate count in a clean room to the tolerances demanded by the density of modern integrated circuits.
Because of the increasing importance of cluster tools, it is becoming commercially important to decrease the time required for each process step on a wafer. In the past, many wafers were processed simultaneously in large furnaces. Because of the high vacuum requirements and mechanical transfer requirements, a cluster tool working module generally processes only one wafer at a time. Although duplicate identical modules can be clustered around a transfer chamber, it is seen that the cluster tool device is essentially a serial processing system and that the process time of each step will have a direct affect on the overall throughput rate.